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Ok, about 3 weeks ago I replaced the DC in board on my wife's 15" G4 1.25 powerbook. When I replaced the mobo I cleaned off all the heat paste and reapplied new generic while silicone based heat paste from Radio Shack. I did not know about Arctic 5 at the time. The computer has worked fine ever since until earlier this week.

It had been on running for a while when my wife noticed a "hissing" noise coming from the computer. She said she could smell a hot, burning smell and the computer was frozen with Safari up. She shut it down, and later tried to reboot it with no luck. When you power it up the grey screen comes up and the spinning clock comes up, but it does nothing, and after a while it goes to a blue screen.

I just removed the mobo again, and am wondering what has failed. There was good coverage of thermal paste on all the spots that Apple used it. The paste on the processor area was wet around the edges - almost like the paste had sizzled the moisture out of itself?

So, what does this sound like to you? Would the white paste versus arctic silver cause a meltdown? How can I test the processor?

Oh - and if you think it's the paste I need to know fast. My mother in law's G4 got the same paste at the same time when I went in and resoldered the dc in jack back into the dc in board that it had broken loose from. I told her not to use it till I figure out what happened!

Sounds like to me you used too much paste on the processor. Too much is as bad as not enough and I suspect that hissing and the wet edges is indicative of burning excess paste. You need to apply paste on an Intel dual core CPU down the middle of the cores in a thin stripe, then put the heat sink on carefully to insure a good bond.